ABSTRACT
In response to DNA damage and genotoxic stress, the p53 tumor suppressor triggers either cell cycle arrest or apoptosis. The G(2) arrest after damage is, in part, mediated by the p53 target, 14-3-3final sigma (final sigma). Colorectal tumor cells lacking final sigma are exquisitely sensitive to DNA damage. Here we analyzed the mechanism of this sensitivity in final sigma(-/-) as compared with final sigma(+/+) human colorectal tumor cells. Exposure to adriamycin resulted in rapid apoptosis only in final sigma(-/-) cells. This was further characterized by caspase-3 activation, p21(CIP1) cleavage, and CDK2 activation. Moreover, Bax was rapidly translocated out of the cytoplasm, and cytochrome c was released in final sigma(-/-) cells. Transient adenovirus-mediated reconstitution of final sigma in the final sigma(-/-) cells led to effective rescue of this phenotype and protected cells against apoptosis. The association of final sigma, Bax, and CDK1 in protein complexes may be the basis for this antiapoptotic mechanism. In conclusion, final sigma not only enforces the p53-dependent G(2) arrest but also delays the apoptotic signal transduction.
Subject(s)
Apoptosis , G2 Phase , Mitosis , Proto-Oncogene Proteins c-bcl-2 , Proto-Oncogene Proteins/metabolism , Tyrosine 3-Monooxygenase/chemistry , Tyrosine 3-Monooxygenase/physiology , 14-3-3 Proteins , Adenoviridae/genetics , Caspase 3 , Caspases/metabolism , Cell Cycle , Cell Line , Cyclin-Dependent Kinase Inhibitor p21 , Cyclins/metabolism , Cytochrome c Group/metabolism , Cytoplasm/metabolism , Cytosol/metabolism , Doxorubicin/pharmacology , Enzyme Activation , Humans , Microscopy, Fluorescence , Mitochondria/metabolism , Phenotype , Precipitin Tests , Protein Binding , Protein Transport , Signal Transduction , Tumor Suppressor Protein p53/metabolism , bcl-2-Associated X ProteinABSTRACT
In the infected host, the Nef protein of HIV/SIV is required for high viral loads and thus disease progression. Recent evidence indicates that Nef enhances replication in the T cell compartment after the virus is transmitted from dendritic cells (DC). The underlying mechanism, however, is not clear. Here, we report that a natural variability in the proline-rich motif (R71T) profoundly modulated Nef-stimulated viral replication in primary T cells of immature dendritic cell/T cell cocultures. Whereas both Nef variants (R/T-Nef) downregulated CD4, only the isoform supporting viral replication (R-Nef) efficiently interacted with signaling molecules of the T cell receptor (TCR) environment and stimulated cellular activation. Structural analysis suggested that the R to T conversion induces conformational changes, altering the flexibility of the loop containing the PxxP motif and hence its ability to bind cellular partners. Our report suggests that functionally and conformationally distinct Nef isoforms modulate HIV replication on the interaction level with the TCR-signaling environment once the virus enters the T cell compartment.